First UVCS Observations of a Comet's Tail

The four image segments on the left are the UVCS observations. Each
section of the image is a composite of several 200 second exposures.
The instrument stays at one of the four heights as the nucleus and the
tail pass through the observed region. The position and size of the
Sun is indicated by an EIT 304 Å image.
Also: High-resolution TIFF image.

Caption:
Comet SOHO-104 (officially designated C/2000 C6), a member of the
Kreutz family of Sun-grazing comets, was first discovered by the Large
Angle and Spectrometric Coronagraph (LASCO) on 9 February 2000. The
group of images shown here illustrate the four heights (6.37, 5.71,
4.57, and 3.27 Solar radii) where the comet was observed by the
Ultraviolet Coronagraph Spectrometer (UVCS) on the following day.

The light detected from the comet has a wavelength of 1216 Angstroms,
referred to as Lyman-alpha. Hydrogen atoms produced by the
dissociation of water vapor outgassed from the nucleus scatter the
Lyman-alpha from the Sun towards the UVCS instrument. As its orbit
takes it closer to the sun, the comet brightens as ice evaporation
increases, and then fades as less material is available for
vaporization.

Since the neutral Hydrogen atoms are "destroyed" (ionized) by
collisions with the surrounding corona, the comet can be used as a
probe to determine the density of of the region of the corona it is
passing through. This is accomplished by measuring the decrease in the
Lyman-alpha flux from the tail to determine the time it takes for the
intensity to drop by half. When the result is combined with the
ionization rate coefficient for Hydrogen (taken from Scholz & Walters,
ApJ 380, 302), the coronal density can be determined.

For more images of comets discovered by SOHO, see the Hot Shot for
SOHO Comet 100.